Long period seismometer



1955 F. E. ROMBERG LONG PERIQD SEISMOMETER 5 Sheets-Sheet l Filed March27, 1961 INVENTOR. Frederick E. Romberg BY Oct. 12, 1965 F. E. ROMBERGLONG PERIOD SEISMOMETER 5 Sheets-Sheet 2 Filed March 2'7, 1961 331 1.53"EFE INVENTOR. Frederick E. Romberg BY A 6 EN T Oct. 12, 1965 F. E.ROMBERG 3,212,057

LONG PERIOD SEISMOMETER Filed March 27, 1961 5 Sheets-Sheet 3 INVENTOR.Frederick E. Romberg BY AGENT United States Patent 3,212,057 LONG PERIODSEISMOMETER Frederick E. Romberg, Dallas, Tex., assignor to TexasInstruments Incorporated, Dallas, Tex., a corporation of Delaware FiledMar. 27, 1961, Ser. No. 98,564 11 Claims. (Cl. 340-17) This inventionrelates generally to vibration detecting seismometers, and moreparticularly to that group of seismometers that convert a physicalvibratory motion into an electrical output signal.

A primary object of this invention is to provide an extremely accuratelong period horizontal motion detecting seismometer.

Another object is to provide a long period horizontal motion detectingseismometer which is accurate in operation and yet extremely durable andrugged in use.

An additional object of this invention is to provide a horizontal motiondetecting seismometer which operates on the pendulum principle, and yetwhich operates at a controlled period that is longer and selectivelyadjustable relative to the period that a corresponding conventionalgravity-type pendulum would have.

A further object of this invention is to provide a relatively shortlength pendulum-type seismometer which will have a period of operationthat would correspond to an extremely long length conventional typependulum.

A further object of this invention is to provide a simple, compact,unitary seismometer which operates to detect long period horizontalmotion, and yet to provide such a seismometer that is quickly and easilyinstalled in the field for operation, and one that is easily adjustedfor misalignment.

A still further object of this invention is to provide a long periodhorizontal motion detecting seismometer in which the geometry of theconstruction of this seismometer is such that it will toleratemisalignment at installation which would prevent conventional longperiod horizontal motion seismometers from functioning accurately.

Certain prior art seismometers are known which are specifically intendedto measure long period horizontal component vibrations, but theseearlier devices were very sensitive to tilt so that it was necessary toinstall them permanently in their properly adjusted position before theywere suited for use. This virtually eliminated their value as portableinstruments. Many of these prior art horizontal seismometers weredisturbed in their operation if subjected to a tilt of as small as 2seconds of arc; the present invention can operate satisfactorily with asgreat a tilt as A2 of a degree.

Horizontal seismometers have been used in the past to detect earthquakesthroughout the world. Detecting earthquakes is a continuous endeavor andmost of the stations that are intended for this purpose are of apermanent nature; however, there has arisen at the present time a needfor a long period seismometer that is extremely rugged and durable andrelatively insensitive to slight tilt so that it may be used as aportable instrument. This need is in establishing a world wide nuclearexplosion detection system in which seismometers are located in arraysto not only accurately determine the location of a large vibration butalso to distinguish between the vibration resulting from an explosionand from an earthquake. This is done by an examination of certaincharacteristics of the long period sections of a visual recordseismogram of an earthqutke and a corresponding seismogram which wastriggered by nuclear explosion.

The present invention utilizes the principle of a vertically extending,horizontally moving pendulum but incorporates sufiicient structure andgeometry to actually 3,212,057 Patented Oct. 12, 1965 increase theperiod of the pendulum in this seismometer beyond the period it wouldexperience as a solely gravity operated pendulum. This advantage isgained by the use of a relatively short length pendulum which is pivotedat a point in vertical alignment with the null position of the mass ofthe pendulum, but which includes a zero length spring which attaches tothe mass at the lower end of the pendulum and also attaches to a point acertain distance above the pivot so that in effect the spring isconsiderably longer than the pendulum itself. This structure incombination with the characteristics of the spring itself and the use offrictionless bearings will permit this seismometer to achieve theresults desired in this invention.

The lateral dimensions of a seismometer constructed according to thisinvention are small and of such shape that the seismometer is adaptablefor use in a bore hole beneath the surface of the earth and may beraised and lowered from its installed position by means similar to thoseused in oil field drilling and exploration operations.

These and other objects of this invention will be apparent from anexamination of the following specification and drawings in which:

FIGURE 1 represents a side elevational view of the long periodhorizontal motion seismometer of this invention;

FIGURE 2 is a fragmentary sectional view of the pendulum of thisinvention showing the manner in which the electrical output signal isgenerated;

FIGURE 3 is a schematic diagram explaining the principle of operation ofthe pendulum-spring combination employed in this invention;

FIGURE 4 is a schematic illustration similar to that shown in FIGURE 3but intended for a theoretical explanation of the effect of verticaltilt on the seismometer of this invention;

FIGURE 5 is a graphical illustration of a comparison between aseismogram or pictorial record of an earthquake versus a nuclearexplosion;

FIGURE 6 is an elevational view of a single portable seismometerinstalled for field use at a pre-selected location;

FIGURE 7 is an outline plan view showing how an array of theseseismometers may be used to completely examine the vibration of theearth due to one of the mentioned causes.

Referring now more particularly to the characters of reference on thedrawing, it Will be observed in FIG- URE 1 that the seismometer assemblyof this invention identified generally at 2 is seen to include .a frame3 having an open area 4 in which is supported a spring loaded shortpendulum 5 and having a flat generally horizontal base 6 which providessupport for the electrical signal generating means 7 which permits areplica of the move ment of the pendulum to be electrically transmittedand remotely recorded. Vertical sides 8 and a top cross member 9 of theframe 3 outline and define the open area 4.

The frame 3 also includes a pair of inwardly projecting bearing supportarms 10, each containing a removable block 11 secured on the arms 10 bymeans of capscrews 12. The pendulum 5 includes corresponding blocks 11::and screws 12a located on outwardly projecting arms 13. Each of theblocks 11 and 11a are installed initially so as to be in verticalalignment with each other. This construction permits the combination ofone block and its screws to act as a clamp to bind a short length oftungsten wire 14 between the block and the corresponding area of arm 13(or 10). The tungsten wires 14 are formed in a generally V-shape toprovide support for the free ends of the small bearing springs 15. Thisconstruction will provide a substantially frictionless bearing in whichpendulum 5 may oscillate when an external force is applied to the framecontaining the pendulum. An upward force is supplied to the pendulum tokeep springs in tension during operation by a Zero length" spring 16which is itself suspended and supported by V-shaped tungsten wires 17which are held in a similar manner to the frame and pendulumrespectively by their mounting blocks 11b and 11c.

The lower end of the pendulum 5 is enlarged at 20 to provide anincreased mass at the lower extremity of the pendulum. Small adjustmentsin the effective weight of the pendulum may be gained by repositioningthe small nuts 21 on the threaded upright 22. At each of three locationson the base 6, there is installed an adjustment screw 23 having aknurled head 24 to permit hand rotation of the screw 23 in eitherdirection in its threaded hole (not shown) to provide a micrometer-typeadjustment for the base 6 and consequently a leveling action for theentire seismometer assembly 2.

In FIGURE 2 the construction of the electrical signal generating unit 7may be observed in more detail. This unit includes two spaced apartgenerator housings 25 made of soft iron. Each housing includes apermanent magnet 26 rigidly attached to each inner end of the housing insuch a manner as to be suspended centrally of the path of movement ofthe circular armature 27 which is integrally attached to the lower endof the pendulum 5 just below its enlarged mass portion 20. Electricalleads 30a and 30b connect opposite ends of pick-up coil 31 and transmitthe signal to a remote recorder or the like. Since the movement of thelower end of the pendulum is very slight (from O to .1" duringearthquakes), the small slack in the leads and their cable housing 30will permit their use in the manner shown. As an alternate signalread-out a photo-electric cell 40 is installed on base 6 in alignmentwith a light source (not shown) at the null position of the pendulum,

In FIGURE 5 a comparison may be had between the seismogram of anearthquake, or natural phenomenon and a nuclear explosion or man-madevibration. FIGURE 5a is a typical visual record of earthquake vibrationwave, and it will be observed that the frequency of certain modes of theprincipal shock wave amplitudes is such that the wave spacings, orperiod is relatively large, for example approximately 15 seconds.Whereas in FIGURE 5b it will be seen that the period or spacing S of thepeak vibration waves of the same modes following a nuclear explosion issubstantially smaller, for example approximately 8 seconds. Theseismometer assembly of this invention is very capable of detecting bothof these long period vibrations in a horizontal direction.

In actual use as shown in FIGURE 6, the small portable seismometerassembly 2 will be installed in a thermally insulated housing 32 afterthe assembly has been placed on the top flat surface of a concretepedestal 33 and leveled by means of adjustment screws 23. The outputsignal from the assembly 2 is carried by the electric cable 30 to aremotely located recorder 34, which may be a portable instrument or maybe truck mounted if desired.

In FIGURE 7 it is shown that this seismometer assembly 2 may be used incombination with a conventional vertical motion detecting seismometerindicated at 35 to provide an array to detect earth movements in anydirection. These arrays may be located throughout the world and theircombined outputs will permit an accurate location of the desired event,be it an earthquake or explosion.

The various elements and the geometry of the construction of thisinvention are so combined that the following advantages of a long periodhorizontal motion detecting seismometer are obtained:

(a) A short length, short movement vertical pendulum having a longperiod,

4 (b) A pendulum type horizontal motion detecting seismometer whoseperiod is relatively insensitive to nominal tilt.

The long period of this seismometer is attained by the specifiedrelationship between the mass of the short length pendulum and by theforce of the restraining spring. This relationship is determined by thegeometry of the pendulum-spring combination, the spring constant of themain spring, and the mass of the pendulum, and this relationship issuch, that, once the pendulum mass is put into motion, the torque due togravity which would tend to return the pendulum to its Zero or nullposition is almost, but not quite balanced by the torque due to thespring tension of the restraining spring which would tend to rotate thependulum in an upward direction. The following mathematical analysiswill illustrate this relationship:

The torque due to gravity '1'.; may be seen by reference to FIGURE 3 andis obtained from the following equation;

Where M is the mass, g the acceleration due to gravity, d the effectivependulum length.

The torque due to the spring tension T (this being a zero length spring)is obtained from the following formula:

T =Fa sin 5 Where a is the height of the upper spring suspension abovethe hinge or bearing line and B is the angle between the vertical andthe line of action of the spring.

The spring force in tension, F, is obtained from the equation:

Where k is the spring constant, r is the extended length of the springat force F, and c is effective or virtual length of the unstressedspring. In a zero length spring, 0 equals Zero, and in a negative lengthspring, 0 is a negative nuumber; in an ordinary or positive lengt spring0 equals a positive number. In this invention a zero length spring ispreferred, but operative units may be made employed slightly positive orslightly negative springs.

From trigonometry the following values of FIGURE 3 may be equated:

sin fi=(b/r) sin T =F(ab/r) sin (I: T =kmb(1c/r) sin Thus the totaltorque T=T +T is given by T=[kab(lc/r) Mgd] sin 5 Then if c=0, and thesystem is adjusted so that kab=Mgd, T becomes zero and the period of thesystem becomes infinite. If Mgd kab, the system becomes oscillatory whensubjected to a small shock or vibration. The academic equation forundamped motion is:

Md =(kabMgd) sin and the period t for small oscillations is:

The sensitivity of the period of this seismometer to nominal tilt in thevertical plane perpendicular to the plane of motion, may be examined byreference to FIG- URE 4 in which 0 is the angle of tilt. The componentof gravity is thus reduced by a factor of the cosine of the angle oftilt, while the spring torque T remains unchanged. This reduces theoriginal period to then the rate of change of the period with tilt(cit/d0) becomes:

dt/d0=1rg sin 0(d) (g cos 0 kab/Md) which is small if 0 is small.

In a practical illustration, if the seismometer has a period of 30seconds, and a pendulum length of 20 cm., then a tilt of as much as 36minutes of arc will increase the period of time by only one second. Infield installations however, even with this portable seismometer it isrelatively easy to quickly set up the instrument to an accuracy ofone-fourth of a degree (15 minutes of arc).

The sensitivity of the displacement of this seismometer from itspendulum null position to tilt may be examined by reference to FIGURE 4in Which 0 is the angle of tilt in the plane of motion of the pendulumin a manner similar to the analysis above relative to the sensitivity ofthe period. Here the gravitational torque T is unchanged, while thespring torque T will be changed.

In actual practice, if there is a tilt from zero to one second of arc,in the seismometer of the previous example, then the zero position willbe shifted by 19 minutes of arc.

Although certain specific embodiments of the invention have been shownand described, it is obvious that many modifications thereof arepossible. The invention, therefore, is not to be restricted exceptinsofar as is necessitated by the prior art and by the scope of theappended claims interpreted in the light of the spirit of thisinvention.

What is claimed is:

1. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a bearing in said supportmeans, a vertically extending pendulum supported by said bearing andadapted to oscillate therein, and a vertically extending spring meansconnecting said pendulum to said frame, to apply to said pendulum atorque opposite and substantially equal to the gravitational restoringtorque on said pendulum.

2. A seismometer according to claim 1 wherein said spring means includesa spring that has substantially a zero length.

3. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a bearing in said supportmeans, a vertically extending pendulum supported by said bearing andadapted to oscillate therein in a vertical plane, and a verticallyextending spring means connecting said pendulum to said frame above saidbearing to apply to said pendulum a torque opposite and substantiallyequal to the gravational restoring torque on said pendulum.

4. A seismometer according to claim 3 wherein said spring means includesa spring that has substantially a zero length.

5. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a base in said frame, asubstantially frictionless bearing in said support means, a verticallyextending pendulum of a length substantially shorter than the height ofsaid frame supported by said bearing and adapted to oscillate therein ina vertical plane, a vertically extending spring means connecting saidpendulum to said frame and opposing the gravitational restoring torqueof said pendulum, and electrical generating means to detect saidmovement of said pendulum supported on said base means and adjacent thelower end of said pendulum.

6. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a substantially frictionlessbearing in said support means, a vertically extending pendulum supportedby said bearing and adapted to oscillate therein in a vertical plane, avertically extending spring means connecting said pendulum to said frameand opposing the gravational restoring torque of said pendulum, andelectrical generating means to detect said movement of said pendulum;said spring means extending a substantial distance above said bearingsupport means to said frame.

7. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a base in said frame, asubstantially frictionless bearing in said support means, a verticallyextending pendulum of a length substantially shorter than the height ofsaid frame supported by said bearing and adapted to oscillate therein ina vertical plane, a vertically extending spring means connecting saidpendulum to said frame and opposing the gravitational restoring torqueof said pendulum and electrical generating means to detect said movementof said pendulum, said electrical generating means comprising agenerator housing attached to said base adjacent the lower end of saidpendulum and containing a permanent magnet, and a pickup coil attachedto the free end of said pendulum for interacting with the magnetic fieldof said permanent magnet.

8. A long period horizontal motion detecting seismometer comprising aframe, bearing support means on said frame, a substantially frictionlessbearing in said support means, a vertically extending pendulum supportedby said bearing and adapted to oscillate therein in a vertical plane, avertically extending spring means connecting said pendulum to said frameand opposing the gravitational restoring torque of said pendulum,electrical generating means to detect said movement of said pendulum,and adjusting means on said frame to level said seismometer in alldirections.

9. A long period horizontal motion detecting seismometer, comprising aframe, a vertically extending pendulum supported by said frame andadapted to oscillate in a vertical plane, and a vertically extendingspring means connecting said pendulum to said frame and opposing thegravitational restoring torque of said pendulum.

10. A long period horizontal motion detecting seismometer, comprising aframe, a vertically extending pendulum, bearing means connecting saidpendulum with said frame and adapted to enable said pendulum tooscillate in a vertical plane when said pendulum is biased upward, and avertically extending spring means connected to said frame and biasingsaid pendulum upward so as to oppose the gravitational restoring torqueof said pendulum.

11. A seismometer according to claim 10 wherein the spring constant ofsaid spring means has a magnitude sufiicient to cause said gravitationalrestoring torque to be substantially equalized.

References Cited by the Examiner UNITED STATES PATENTS 1,774,379 8/30Jones 340-17 2,390,328 12/45 Roberts 340-17 2,636,160 4/53 Loper et al.340-17 3,026,428 3/62 French 340-17 SAMUEL FEINBERG, Primary Examiner.KATHLEEN H. CLAFFY, NEIL C. READ, Examiners.

1. A LONG PERIOD HORIZONTAL MOTION DETECTION SEISMOMETER COMPRISING AFRAME, BEARING SUPPORT MEANS ON SAID FRAME, A BEARING IN SAID SUPPORTMEANS, A VERTICALLY EXTENDING PENDULUM SUPPORTED BY SAID BEARING ADADAPTED TO OSCILLATE THEREIN, AND A VERTICALLY EXTENDING SPRING MEANSCONNECTING SAID PENDULUM TO SAID FRAME, TO APPLY TO SAID PENDULUM ATORQUE OPPOSITE AND SUBSTANTIALLY EQUAL TO THE GRAVITATIONAL RESTORINGTORQUE ON SAID PENDULUM.